1. Technical Field
The present invention relates generally to image sensors, and more particularly, to a method of driving an image sensor for reduced noise and with blooming current flow.
2. Description of the Related Art
Image sensors convert optical images into electrical signals and are widely used in electronic devices, such as digital cameras, mobile phones, etc. The image sensors are generally divided into a charge-coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
The CCD image sensor consumes more power than the CMOS image sensor, but the CCD image sensor has less noise and better image quality in comparison with the CMOS image sensor. Since the CMOS image sensor may be produced using conventional CMOS fabrication, the CMOS image sensor is easily integrated to a peripheral system for performing operations such as amplification and signal processing, resulting in reduced manufacturing cost. Further, the CMOS image sensor has higher operating speed than the CCD image sensor.
A typical image sensor includes a pixel array including a plurality of pixels for example arranged as a two-dimensional matrix. Each pixel includes a respective photo-detector (such as a photodiode for example) for generating electric charge from received light. Each pixel also includes other devices such as transistors for converting such electric charge into an electrical signal. A pixel of the typical image sensor may have a three-transistor structure, a four-transistor structure, a structure with some transistors being shared by a plurality of pixels, or the like.
A dynamic range (DR) of an image sensor represents the capability of the image sensor to distinguish between light and shade of a subject. For example, the dynamic range may be represented as 20*log(S/N) in a unit of decibel (dB), where S represents a maximum level (i.e., a saturation level) of a signal recognizable by the image sensor, and N represents a noise level of the image sensor. Thus, the dynamic range of the image sensor may be extended by decreasing the noise level N or by increasing the saturation level S.
A linear image sensor outputs an image signal linearly proportional to an intensity of received light such that post-processing for the image signal of the linear image sensor may be easily implemented. A dynamic range of the linear image sensor, however, may be limited to below about 75 dB. Various methods have been proposed to increase the dynamic range of the linear image sensor. However, size and cost of the image sensor are generally increased by such proposed methods since the charge storage capacity of a photo-detector (e.g., a photodiode) in a pixel should be increased resulting in a limit to the increase of the dynamic range. Thus, the linear image sensor is not suitable for a device desired to have a small size.
A logarithmic image sensor uses a concept of sub-threshold conduction of a MOS (metal oxide semiconductor) transistor. Since the logarithmic image sensor generates an image signal that is logarithmically proportional to the intensity of the received light, the logarithmic image sensor may have a wider dynamic range of above about 100 dB.
The logarithmic image sensor, however, may have a long RC response time when the intensity of the received light is low. Furthermore, the logarithmic image sensor is more sensitive to noise since the logarithmic image sensor outputs an electrical signal that is compressed with respect to the received light.
Generally, an active pixel in a pixel array of an image sensor includes a transfer gate formed on a semiconductor substrate between a photo-detector and a floating diffusion region formed in the semiconductor substrate. The image sensor operates according to an integration mode and a read mode defined by a switching operation of such a transfer gate.
A typical logarithmic image sensor generates an image voltage signal that is proportional to an amount of electric charge generated by the photo-detector during the integration mode. If the amount of electric charge generated in the photo-detector (such as a photo-diode for example) during the integration mode is higher than a charge storage capacity of the photodiode, a portion of such generated electric charge is discharged as a blooming current from the photodiode.
The typical logarithmic image sensor drives the transfer gate with a voltage level near a threshold voltage during the integration mode for sub-threshold conduction through the transfer gate of the blooming current. In that case, noise may be increased from dark current flowing below the transfer gate, especially at a high temperature.